The use of aqueous limestone slurries to scrub flue gasses for the removal of sulfur dioxide is well known in the art. However, the reactivity of the limestone is such that it allows an unacceptable level of SO.sub.2 slippage, i.e., SO.sub.2 remaining in the scrubbed gas. In view of the relatively low reactivity of limestone it is common practice to calcine the limestone, drive off CO.sub.2, and convert the limestone into CaO. When the latter is slurried in water, it forms Ca(OH).sub.2, which in slurry form exhibits advantages over limestone slurry in a number of areas, principally in reduced SO.sub.2 slippage and in more complete utilization of the scrubbing reagent (i.e., a greater percent conversion of the calcium into CaSO.sub.3).
The calcining of limestone is heat intensive. Kirk-Othmer gives a range of 31/4 to 10 million Btu of thermal energy required per ton of CaO produced. Over the past six years, the cost of fuel has greatly increased, adding to the desirability of "activating" limestone in a way less fuel-sensitive.